This invention relates generally to size classified small particles, and more specifically to size classification of electrostatographic toner particles, and their use admixed with carrier particles in the electrophotographic copying process.
In electrophotography, a photoconductor is charged and then exposed imagewise to light. In the area of the photoconductor exposed to light, the charge dissipates or decays while the dark areas retain the electrostatic charge.
The difference in the charge levels between the areas exposed to light and the dark areas produces electrical fields therebetween. Thereafter, the resultant latent electrostatic image on the photoconductor is developed by depositing small colored particles, which are known as toner particles, having a charge so as to be directed by the electrical fields to the image areas of the photoconductor to develop the electrostatic image.
A number of means are known for developing the latent electrostatic image by the application of the toner particles. One of these is known as cascade development and is described in U.S. Pat. No. 2,638,552 to Wise. Another means is known as the magnetic brush process. This method is described in U.S. Pat. No. 2,874,063 to Greig.
In each of the cascade and magnetic brush development processes, a two component developer material is utilized. The developer material comprises a mixture of small toner particles and relatively large carrier particles. The toner particles are held on the surfaces of the relatively large carrier particles by electrostatic forces which develop from the contact between the toner and carrier particles producing triboelectric charging of the toner and the carrier to opposite polarities. When the developer material is moved into contact with the latent electrostatic image of the photoconductor, the toner particles are attracted to the latent image.
The toner and carrier particles of the developer material are specially made and processed to that the toner obtains the correct charge polarity and magnitude of charge to insure that the toner particles are preferentially attracted to the desired image areas of the photoconductor. The toner particles are then transferred electrostatically to the desired copy sheet, after which the transferred image of toner particles is fused by heat and/or pressure to produce the final product of a fused copy of the desired image.
One of the problems encountered is to provide the best possible quality of a final image on the copy sheet. This is generally referred to as copy quality. Copy quality includes such things as image clarity, i.e., clear delineation of lines; uniform darkness of the image areas; background quality, i.e., grayness or lack of it in the background areas; and other somewhat intangible features that go toward making a good "quality" copy.
Other factors that merit consideration in the developing process vis-a-vis toner is the overall utilization of toner per copy. Of course from an economic point of view the less toner used per any given image the better. Also in a system in which unused toner is cleaned from the air by use of a filter, it is important to minimize the amount of unused toner to thereby extend the life of the filter.
Further, when heat fusing is used it is desirable to provide an image that will have the best possible heat transfer characteristics to minimize the amount of heat needed to fuse the image. This is important not only from an energy point of view, but also with more rapid heat transfer by the toner, the fusing time or temperature can be reduced.
All of these factors play important roles in developing an optimum toner particle.
One of the principal contributing characteristics of the toner particles in achieving optimum results in the above-noted areas in the size and size distribution of the toner particles. This fact in itself is well known, and there have been several prior art proposals for various systems of toner particle classification.
U.S. Pat. No. 3,674,736 to Lerman et al discloses pigmented polymer particles suitable "for use as toner . . . and as developers for electrostatic process," and the method of making such toners. This patent claims material having an average particle diameter within the range of from about 1 to 30 microns (NMD) and a GSD of less than about 1.5. By extrapolation and the use of Gaussian distribution this can be related to a particular size distribution.
German Offenlegungsschrift No. 2,522,771 (unexamined published patent application) filed May 22, 1975 and published Dec. 11, 1975 assigned to Xerox, discloses toner particles which essentially have the same distribution as those of the Sherman et al patent. This German reference discloses toner with a size distribution by number or population wherein less than 30% of the particles are less than 5 microns, about 25% are between 8 and 12 microns, and less than 5% are greater than about 20 microns. This German reference also discloses a fine index ratio of less than about 2.50 and a coarse index ratio of less than about 1.50.